METHOD FOR BURNING OF MUNICIPAL WASTES AND THE USE OF THE ASH PRODUCED IN THE BURNING

Abstract: The invention relates to a method for burning municipal wastes. Solid, unsorted wastes (1) are pyrolyzed on a grate (4, 5), primary combustion air being introduced from below it, and the forming gas phase, to which secondary combustion air is added, is conveyed to a combustion chamber (8) to burn the combustible components of the gas. The essential idea in the invention is that, by means of air feed, the combustion temperature in the combustion chamber (8) is adjusted to at least 850 DEG C, that the ash separating from the solids burning on the grate (4, 5) is caused to melt at least in part so that, upon cooling, it will form granular, purely inorganic particles (9), and that by means of the infeed of secondary air, a turbulent mixing of air and pyrolysis gases is produced in the grate area. At the said temperature, complete combustion of the organic matter is achieved, and the forming pumice-like ash is usable, for example, as a filler material for earth construction.

Description: Method for burning of municipal wastes and the use of the ash produced in the burning The present invention relates to a method for burning municipal wastes, in which method solid wastes are pyrolyzed on a grate by introducing primary combustion air from below the grate, and in which method the formed gas phase, to which secondary com¬ bustion air is added, is conveyed to a combustion chamber for the burning of the combustible components present therein. The principal alternatives for the treatment of municipal wastes have been the transportation of the wastes to dumps or their disposal by burning. Owing to the centralization of dumps, the transportation costs have increased, and the re¬ quirements for the insulation of dumps and for the treatment of the leachate have also increased the costs. This has improved the relative competitiveness of the burning of waste, which is further promoted by the energy recoverable from the burning. On the other hand, environmental hazards due to combustion residues have constituted a problem in the burning of wastes on a grate. The burning produces large amounts of ash, typically approx. 250 kg of furnace ash, approx. 30 kg of filter ash, and approx. 0.7 kg of fly ash per one metric ton of waste; these consist of only partly burned waste and contain large amounts of highly toxic organic compounds, mainly dioxins and other polyaromatic hydrocarbons, as well as heavy metals. In nature these compounds concentrate in food chains and can be found, for example, in agricultural produce from the vicinity of in¬ cineration plants. A more environment-friendly procedure has been to burn waste in a fluidized-bed furnace, especially if cleansing of the flue gases has been linked thereto. This method, however, has the deficiency that only approx. 50-70 % of the wastes are suitable for being burned in a fluidized bed; the rest of the wastes must be separated and treated in some other manner. It is also known to burn wastes of a low thermal value mixed with a fuel with a higher thermal value, such as fuel oil or natural gas, or with wastes resembling them, such as waste oil or solvents. Thereby a sufficiently high temperature and com¬ plete burning are accomplished, but for cost reasons the method has not been used for treating conventional municipal wastes but for treating only hazardous wastes for which other treat¬ ment methods are not suitable. An object of the present invention is to provide a method based on burning on a grate, by which method unsorted municipal wastes can be disposed of in a more economical and more environment-friendly manner. The method according to the inven¬ tion is characterized in that the combustion temperature in the combustion chamber is adjusted by means of air feed to at least 850 °C, that the ash separating from the solids burning on the grate is caused, at least in part, to melt, in which case it, upon cooling, will form granular particles which contain only inorganic matter, and that the said secondary air is fed into a gas flow passing the grate in such a manner that a turbulent mixing of gases will occur in the grate area. Burning of wastes in accordance with the invention presupposes that the combustion temperature is continuously sufficiently high, at least the said 850 °C, that the combustion conditions on the grate and in the combustion chamber are stationary also in other respects. Ash formed at the said temperatures in part melts and in part becomes sintered into small drops, so that the molten phase is on the surface of sintered particle nuclei. In this case the ash does not produce much dust, a factor which enables secondary air to be mixed in a turbulent manner with the gas phase produced by pyrolysis and containing combustible components; this, in turn, will ensure effective mixing to form a homogenous gas mixture which, when burning in the combustion chamber, will maintain the desired high temperature. The lack of dust in the ash presupposes that there are silicates pres¬ ent, for example silicate of sodium, potassium or aluminum; municipal waste, however, always contains these, for example, in the form of dust and sand. According to the invention, the primary air is preferably fed to the upstream section of the grate, which may be tilted, and the secondary air is fed to the downstream section of the grate, on which the final burning of the solids takes place and on which the ash is at least in part in molten state. The sec¬ ondary air may be fed into the mixing zone of pyrolysis gases and air in the downstream section of the grate from a feeding conduit which at the same time serves as a wall separating the waste-containing fuel silo and the combustion chamber from each other. In this case the feed conduit separates from each other the pyrolysis zone in the silo and the gas combustion zone made up of the combustion chamber, the mixing zone of gas and air between these being located substantially at the tip of the conduit. This arrangement enables heterogenous wastes of vary¬ ing composition and moisture content to be burned homogenously and therefore completely. Preliminary testing of the method according to the invention and analyses of the combustion residues have shown that the quantity of grate ash formed is approx. one-half of the prev¬ ious quantity, and no dioxin or any other non-combustible or¬ ganic matter is left in the ash. The structure of the ash re¬ sembles that of pumice-stone, the ash is obtained in the form of small granules having a diameter of approx. 1-5 mm, and being light in weight, non-toxic, and water insoluble, it is usable, for example, as a filler material for earth construc¬ tion. The invention facilitates the treatment of flue gases leaving the combustion chamber, the treatment typically including cleansing by means of a dust separator, as well as acid and alkaline scrubbing steps, the amounts of solid waste to be separated and the liquid filtrate are reduced, and the purified flue gas released into the atmosphere is cleaner with respect to solid impurities. The maintenance of the high combustion temperature required by the invention is promoted by effective pre-heating, preferably to at least 800 °C, of the secondary air to be fed in. In case the combustion temperature of the waste should for some reason momentarily drop to too low a level, the combustion chamber may be provided with an auxiliary burner to provide the required additional heat. As was stated, a temperature of at least 850 °C is an indispensable condition for complete combustion of dioxins and other organic compounds. The invention relates not only to a method of burning wastes but also to the use of the granular ash particles, formed in the combustion, made up of only inorganic matter, and having a diameter of approx. 1-5 mm, as a filler material for earth construction. Owing to their thermal insulation capacity the particles are especially suitable for groundfrost insulation in road-beds. The invention is described below in greater detail with the help of an example, with reference to the accompanying drawing, which depicts schematically an apparatus for the burning of wastes by the method according to the invention. In the apparatus according to the invention, unassorted munici¬ pal wastes 1 are burned which typically contain paper, card¬ board, plastics, food scraps, dust, sand, glass, ceramic mate¬ rial and metal, the quantities of which may vary according to the origin of the wastes. The wastes 1 may be preliminarily comminuted or standardized in particle size, and they are fed into the fuel silo 2 in such a manner that their surface level 3 in the silo remains within a predetermined range. The wastes 1 fed into the silo 2 rest on a grate in the lower section of the silo, the grate comprising an upstream section 4 tilted in the waste flow direction and a horizontal downstream section 5. Primary combustion air is introduced into the wastes 1 from below the grate 4, and secondary combustion air is fed via a conduit 6 past the column of waste in the silo. In addi¬ tion, the apparatus comprises a feed conduit 7 for tertiary air, via which additional air can be fed into the combustion chamber 8 subsequent to the silo 2 and the grate 4, 5. The feed conduit 6 for secondary air forms a wall separating the silo 2 and the combustion chamber 8 from each other, the distance of its lower edge, i.e. the tip of the conduit, from the grate 5 below being approx. 30 % of the distance between the silo wall and the conduit at the upstream section of the grate. The said distance is relatively smaller than in prior known apparatuses, and its purpose is to separate from each other the process stages taking place in the silo 2 and the combustion chamber 8. For adjustment of the geometry of the feed conduit 6, it can be made movable in the lateral and ver¬ tical directions. When temperature rises in the wastes 1 in the silo 2, first the moisture is homogenized and thereafter the waste dries. The drying turns, without a clear limit, into pyrolysis in which the wastes decompose into coke and gaseous components. The gas phase formed in the pyrolysis zone, which flows through the coke into the mixing zone for secondary air and further into the combustion chamber 8, contains the water vapor separated from the wastes and distributed homogenously into the gas phase, as well as the gas formed in the pyrolysis, which con¬ tains carbon monoxide and organic components, such as turpen¬ tine, organic acids, and decomposition products of phenols and polyaromates. The gas phase also contains dioxins, which are formed from the aromatic compounds and chlorine present in the wastes. What is essential in the pyrolysis is that it takes place entirely in the silo 2 before the slot between the tip of the secondary-air conduit 6 and the grate 5, and thus separate from the combustion occurring in the combustion chamber 8. The coke formed in the pyrolysis zone, and the ash along with it, travel to the horizontal section 5 of the grate, where complete combustion of the coke takes place. It is essential in terms of the invention that the combustion temperature in the combustion chamber 8 is at least 850 °C; this is a prerequisite both for the complete combustion of coke and for the decomposi¬ tion of the formed dioxins and other chlorinated polyaromates. The temperature of the ash left from the coke on the grate 5 is approx. 1000 - 1050 °C, at which the ash at least in part melts and forms drops which, upon cooling, form pumice-like particles having a diameter of approx. 1-5 mm. The formed ash particles 9 accumulate under the grate 5 on the bottom of the silo. The at least partly molten ash formed from municipal wastes is charac¬ terized in that it does not notably form dust; this allows secondary air, pre-heated to approx. 800 °C, to be fed from the conduit 6 into the mixing zone at the downstream section 5 of the grate in such a manner that the gas phase separated in the pyrolysis and the air are mixed turbulently to form a homoge¬ nous mixture which, in turn, maintains stationary combustion conditions and a high combustion temperature. The combustion process is preferably monitored and controlled by using a com¬ puter which regulates the feeding in of combustion air to main¬ tain constant the combustion temperature in the combustion chamber. In case the burning of the waste does not alone suf¬ fice to maintain the required temperature, the temporarily required additional heat can be generated by using an auxiliary burner 10 located in the combustion chamber 8 and using gas or oil. As a rule, however, the process according to the invention operates on the basis of only the combustion heat released from the wastes . In the burning of wastes according to the invention, a substan¬ tially complete combustion of the organic material present in the wastes and a low dust content in the flue gas leaving the combustion chamber 8 are achieved. The feeding in of combustion air is proportioned so that the flue gas leaving the combustion chamber will retain oxygen in an amount of approx. 8-15 %, preferably approx. 11-13 %. The flue gas will contain dust at maximum 100 mg/m 3, in the best case less than 40 mg/m3. The flue gases leaving the combustion chamber 8 travel to a heat exchanger 11, where they are cooled to a temperature below 200 °C. The recovered heat is used, for example, for district heating. The cooled flue gas travels further to a dust filter 12, which separates 85-95 % of the dust into a container 13. This dust is a hazardous waste; however, it can be recycled to the combustion stage. The flue gas leaving the dust filter 12 travels to the gas scrubber 14, where it is first moistened with water and thereafter scrubbed with acid and alkali solu¬ tions, which are fed in via conduits 15 and 16. The overflows from the acid and alkali scrubbing steps are directed via conduits 17 and 18 to a mixing container, where they neutralize each other and from where the neutralized waste solution trav¬ els to a filter 20, where the solids 21 are separated and from where the remaining solution 22, which mainly contains sodium salts, is removed to the drain. The flue gases 23 cleansed in the scrubber 14 can be released into the atmosphere. Example The invention was tested in a pilot apparatus in accordance with the above. The results obtained in several trial runs have been converted below numerically to correspond to an apparatus with a capacity of 1000 kg of waste/h, which is sufficient for the disposing of the wastes produced by a community of 20,000 inhabitants. The dry matter content of the wastes was 60 %, and according to an elemental analysis the dry matter (600 kg/h) contained carbon 43.0 %, oxygen 29.0 %, hydrogen 6.0 %, chlorine 0.8 %, nitrogen 0.4 %, sulfur 0.08 %, and ash 21.0 %. At the slot between the secondary-air feed conduit and the grate the flow rate of the gas produced in the pyrolysis was at its highest approx. 15 m/s and on average, calculated over the entire cross sectional area of the slot, approx. 6 m/s. The amount of flue gas leaving the combustion chamber was 6448 m 3/h, out of which 900 m3/h was water vapor. The oxygen content of the flue gas, calculated from dry gas, was 11.0 %. The flue gas contained fly ash 40 mg/m , in which the portion 3 of heavy metals was 4500 μg/m . Organic material (loss by com- 3 bustion) amounted to 25 mg/m , and this included chlorophenols 0.5 μg/m 3, chlorobenzenes 22.5 μg/m3, and PCB 0.1 μg/m3. Ash accumulated under the grate at a rate of 125 kg/h. Accord¬ ing to measurements, the loss by combustion was 0, i.e. within the limits of the measuring precision there was no unburned organic matter left in the ash. The amount of ash separated by the dust filter was 190 g/h, which contained organic matter (loss by combustion) 90 g/h and heavy metals 20 g/h. In the flue gas leaving the filter, fly 3 ash 6 mg/m and organic matter 3 mg/m were measured. The composition of the flue gas leaving the flue gas scrubber was nitrogen 80.2 %, oxygen 11.0 %, and carbon dioxide 8.7 %. The amount of carbon monoxide measured was 35 mg/m , of oxides 3 of sulfur as S0 2 5 mg/m , of oxides of nitrogen as N0 2 3 3 3 85 mg/m , hydrogen chloride 8 mg/m , fly ash 1 mg/m , heavy 3 3 metals 90 μg/m , and organic matter 500 μg/m , in which PCB 3 3 amounted to 0.04 μg/m and dioxins to 0.00002 μg/m . Water was fed into the scrubber at a rate of approx. 520 kg/h to moisten the flue gas. The alkaline scrubbing liquid used was a sodium carbonate solution in which the amount of Na2CC>3 was 2.3 kg/h. The solution leaving the scrubber contained Me n SO χ 900 g/h as Na 2 S0 3 , Me n NO χ 1000 g/h as Na 2 N0 3 , and MeCl 1000 g/h as NaCl. The Me was mainly sodium. Compounds of Ca, Mg, Al, Fe, Si and P, etc., carbon and hydrocarbons, among others, were found in the solids separated from the solution by the filter. For an expert in the art it is clear that the various embodi¬ ments of the invention are not limited to those presented above as an example but may vary within the accompanying claims.

Claims: Claims 1. A method for burning municipal waste (1), in which method solid wastes are pyrolyzed on a grate (4, 5) by feeding in primary combustion air from below the grate, and in which method the forming gas phase, to which secondary combustion air is added, is conveyed to a combustion chamber (8) to burn the combustible components present in it, characterized in that the combustion temperature in the combustion chamber (8) is ad¬ justed by means of air feed to at least 850 °C, that the ash separating from the solids burning on the grate (4, 5) is caused to melt at least in part, in which case, upon cooling, it will form granular particles (9) containing only inorganic matter, and that the said secondary air is fed into a gas flow passing the grate, so that turbulent mixing of the gases will occur in the grate area. 2. A method according to Claim 1, characterized in that the primary air is fed to the upstream section (4) of the grate and the secondary air to the downstream section (5) of the grate, on which the combustion of the solids is completed and on which the ash is at least in part in molten state. 3. A method according to Claim 2, characterized in that the feed conduit (6) for secondary air forms a wall between the fuel silo (2) which contains wastes and the combustion chamber (8), the wall separating the pyrolysis zone and the gas-phase combustion zone from each other and turbulent mixing of the air and the pyrolysis gases taking place at its tip. 4. A method according to any of the above claims, charac¬ terized in that the combustion temperature in the combustion chamber (8) is maintained constant during the process by regu¬ lating the temperature and/or the rate of the combustion air fed in. 5. A method according to any of the above claims, charac- terized in that the secondary air to be fed in is heated to a temperature of at least 800 °C. 6. A method according to any of the above claims, charac¬ terized in that the combustion air is fed in so that the oxygen content in the flue gases leaving the combustion chamber (8) will be 8-15 %, preferably 11-13 %. 7. A method according to any of the above claims, charac¬ terized in that the dust content in the flue gases leaving the 3 combustion chamber (8) is at maximum 100 mg/m , preferably at 3 maximum 40 mg/m . 8. A method according to any of the above claims, charac¬ terized in that the flue gases which have left the combustion chamber (8) are cleansed in a dust separator (12) and are thereafter scrubbed in a plurality of steps for a selective separation of different impurities. 9. The use of granular ash particles (9) made up of only inorganic matter, having a diameter of approx. 1-5 mm, formed in the method according to any of the above claims, as a filler material for earth construction. 10. The use, in accordance with Claim 9, of ash particles as groundfrost insulation in a road-bed or the like.

Inventors: SAIHA ERIK, SAIHA, ERIK
Applicants: AUVINEN KYOESTI, SAIHA ERIK, AUVINEN, KYOESTI, SAIHA, ERIK
Application year: 1994
Main classification: F23G5
IPC codes: F23G5